PMOS Metal Gate Work Function Tuning via Low-Temperature Oxidation
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Solution Overview
Problem
Attaining desired effective work functions for metal gates in PMOS transistors in advanced integrated circuits without significantly increasing fabrication cost and complexity has been problematic.
Innovation Solution
A process is developed to form PMOS transistors with metal replacement gates, where low temperature oxidation processes are used to increase the effective work functions of the gate work function metal layers from less than 4.8 eV to above 4.82 eV, involving various methods such as thermal oxidation in a steam ambient, exposure to oxygen and hydrogen plasmas, and exposure to an electrolyte solution with ionized oxygen radicals and hydrogen ions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional metal gates are used in PMOS transistors, then fabrication cost and complexity are controlled, but desired effective work functions above 4.82 eV cannot be attained
Solution Approach 1:
The patent applies parameter changes by modifying the effective work function of metal gates through controlled oxidation processes. Specifically, metal layers (such as titanium, tantalum, or tungsten) are oxidized at low temperatures (below 400°C) to form metal oxides or sub-oxides, which adjusts the work function to the desired range above 4.82 eV for PMOS transistors. This chemical transformation changes the electrical properties without requiring complex fabrication processes
Solution Approach 2:
The patent employs strong oxidants in the form of oxygen plasma or steam ambient to accelerate the oxidation of metal gates at low temperatures. The oxygen plasma provides highly reactive oxygen species that rapidly oxidize the metal surface, achieving the desired work function adjustment without requiring high thermal budgets. This accelerated oxidation process enables precise control of the effective work function while maintaining fabrication simplicity
2Manufacturing precision
If high temperature oxidation is used to increase effective work function, then desired work function range is achieved, but thermal budget is exceeded
Solution Approach 1:
The patent replaces thermal oxidation (heat-driven) with plasma-based oxidation (chemistry-driven). By using oxygen plasma or steam ambient, the oxidation process occurs at low temperatures (below 400°C) rather than requiring high thermal budgets. The plasma provides reactive oxygen species that enable oxidation at reduced temperatures, substituting thermal energy with chemical energy from plasma reactions
Solution Approach 2:
The patent utilizes phase transitions of water (liquid to vapor to plasma) to achieve low-temperature oxidation. Steam ambient provides water vapor that can oxidize metal surfaces at low temperatures, while oxygen plasma represents a fourth state of matter that delivers highly reactive oxygen species. These phase transitions enable oxidation processes that bypass the need for high thermal budgets while achieving the desired effective work function
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process effectively raises the effective work function of PMOS transistors to the desired range, improving on-state current densities while reducing electrical resistance and passivating the interface between the metal gates and gate dielectric layers.
Implementation Method 1
oxygen atoms in the work function metal layers may have a distribution of at least 1×1015 atoms/cm2 within 1 nanometer of the top surfaces of the gate dielectric layers
Implementation Method 2
The work function metal layers are oxidized at low temperature to increase their effective work functions toward the desired PMOS range above 4.82 eV
Implementation Method 3
Hydrogen atoms may also diffuse to an interface between the work function metal layers and underlying gate dielectric layers
Implementation Method 4
A second embodiment of the low temperature oxidation process includes exposure to a plasma containing oxygen and hydrogen
Implementation Method 5
A fourth embodiment of the low temperature oxidation process includes exposure to an electrolyte solution containing ionized oxygen radicals and hydrogen ions
Data Source
AI summary
A process is disclosed of forming metal replacement gates for PMOS transistors with oxygen in the metal gates such that the PMOS gates have effective work functions above 4.85. Metal work function layers in the PMOS gates are oxidized at low temperature to increase their effective work functions to the desired PMOS range. Hydrogen may also be incorporated at an interface between the metal gates and underlying gate dielectrics. Materials for the metal work function layers and processes for the low temperature oxidation are disclosed.